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Introduction to Sodium-Potassium-ATPases

The sodium-potassium-ATPase, also known as the Na⁺-K⁺ pump or the sodium pump, is the protein responsible for the ATP-dependent coupled transport of sodium and potassium ions across the plasma membrane. The Na⁺-K⁺ pump is found in all animal cells and is a major force in maintaining the concentration gradients of these ions across the membrane. These gradients provide energy for several cellular functions including control of membrane potential, cell size, and pH homeostasis, and nutrient uptake. Each cycle of ATP hydrolysis, the protein transports three Na⁺ ions out of the cell and two K⁺ ions across the plasma membrane into the cell. In addition to its role as a transport protein, the sodium-potassium-pump has also been shown to act as a receptor for cardiotonic steroid signalling. The sodium-potassium pump was first described in 1957 by Jens C. Skou and he was awarded the Nobel Prize in Chemistry in 1997 for this discovery.

Structure

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The Na⁺-K⁺ pump is a P-type ATPase with a structure similar to the H⁺-K⁺-ATPase[1] and the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA)[2]. Overall, the structure of the sodium-potassium-pump is a transmembrane protein with three subunits labeled α, β, and FXYD.

α-Subunit

The α-subunit is the largest subunit and contains the binding sites for Na⁺, K⁺, and ATP. This subunit is composed of 10 transmembrane α-helices (M1-M10). These helices are centered around a three helix bundle formed by M4-M6. The binding sites for K⁺ and Na⁺ are located within the transmembrane helices. Additionally, there are located on the cytoplasmic face of the membrane: the actuator domain (A), the nucleotide-binding domain (N), and the phosphorylation domain (P). There are 4 known isoforms of the α-subunit, but even the two most divergent isoforms share 78% sequence identity. The majority of structural diversity among the isoforms occurs at the N-terminus, the first extracellular loop, and the third cytosolic domain. This diversity can influence the rate ion transport and the ability to act as a signaling receptor.

β-Subunit

The β-subunit is a single spanning membrane protein with a transmembrane α-helix and a glycosylated extracellular domain. This subunit uses a to bind to the M7 and M10 helices of the α-subunit within the lipid bilayer. These residues also make contact with a cholesterol molecule, the presence of which is necessary for ion transport to occur. Contact between the α and β subunits also occurs at various residues in the extracellular domains. It has important roles in targeting the protein to the membrane and providing stability. It also has a role in providing binding specificity for potassium ions.

FXYD Subunit

The FXYD subunit, sometimes known as the γ-subunit, is an accessory regulatory protein comprised of a transmembrane α-helix and an extracellular domain (which is not shown in this structure). Regulation of ion pumping action by FXYD has been shown to be tissue and isoform specific.

Mechanism of Na/K Pumping

The sodium-potassium pump transports cations across the membrane using what is the "alternative access" model, in which the protein alternates between two conformations, E1 and E2. The crystal structure of the E2 conformation in its phosphorylated state has been reported. No crystal structure is available for the E1 structure, but many aspects of it have been predicted based on homology to SERCA. In the E1 state, ATP is bound at the N domain and high affinity Na⁺ binding sites are open to the cytosol. Binding of three Na⁺ ions causes a conformational change that rotates the N domain so that the γ-phosphate of ATP is positioned near the phosphorylation site of the P domain. ATP is then cleaved and the γ-phosphate is transferred to D376. The A domain then rotates around a horizontal axis large translation and a kink in the first transmembrane helix. This action closes the cytosolic "gate" of the protein, leaving the three Na⁺ molecules momentarily occluded. ADP is also released at this time. This state is known as E1-P and is a rather high energy state. As a result, E1-P rapidly relaxes to E2-P through a conformational change which kinks transmembrane helices 5 and 7 and also opens the extracellular "gate." In this state, the affinity for Na⁺ ions is reduced, allowing them to dissociate into the extracellular environment.

References